USING DEEPWATER DEPOSITIONAL SYSTEMS TO CHARACTERIZE THE THREE-DIMENSIONAL GROWTH OF STRUCTURES WITH EXAMPLES FROM THE NIGER DELTA

Growth strata, sediments deposited during active deformation, record the growth history of a structure and provide constraints on the timing and kinematics of its growth. Deepwater channel and fan systems within the growth strata respond to growing structures with changes in location and sinuosity. We show that 3D geometries of growth strata and patterns of channel systems characterize the full cycle of structural growth, from initiation to cessation, and give key insights into the kinematics of lateral structural growth.

We define two end members of 3D structural growth: structures that growth with fixed edges, and structures that grow through lateral propagation. We build kinematic models to predict the response of channel systems to these end members and use these patterns to determine structural growth in the outer fold-and-thrust belt in the Niger Delta. In both cases, we see the initiation of growth with channel systems deflected by the emergence of bathymetry. In structures that grow with fixed edges, channels maintain relatively fixed positions throughout growth resulting in vertically stacked channel deposits. In structures that grow through lateral propagation, channels are deflected farther outward from the crest of the structure during growth resulting in laterally stacked channel deposits. Both end members record the cessation of growth with channels migrating toward the crest of the structure until all accommodation space is filled and they are no longer affected by changes in bathymetry. We compare our results to isopach and structure contour maps. These maps allow us to compare paleo- and current geometries to determine styles of structural growth. Results indicate that structures in close proximity and individual structures can grow through different end-member mechanisms.

Methods to constrain the timing and kinematics of structural growth give important insights into structural trap evolution and reservoir distribution. For example, if a structure grows through lateral propagation, its trap size and fetch area will increase throughout growth and turbidites, the main deepwater reservoirs, will be incorporated into the trap. However, if a structure grows with fixed edges it maintains a fixed trap size and fetch area and syntectonic channel deposits are generally not incorporated into the trap.